Finned tubes



M. C. PETERS FINNED TUBES Jan. 15,1963

4 Sheets-Sheet 1 Filed Nov. 18, 1958 INVENTOR.

Marfin Charles Perers ATTORNEY M. C. PETERS Jan. 15, 1963 FINNED TUBES 4 Sheets-Sheet 2 Filed Nov. 18, 1958 INVENTOR. Marrin Charles Pe'iers ATTORNEY Jan. 15, 1963 M. c. PETERS 3,073,385

FINNED TUBES Filed Nov. 18, 1958 4 Sheets-Sheet a INVENTOR. Marfin Charles Peters ATTORNEY M. C. PETERS FINNED TUBES Jan. 15, 1963 4 Sheets-Sheet 4 Filed NOV. 18, 1958 INVENTOR. Martin Charles Perers ATTORNEY United States Patent Office 3,073,385 Patented Jan. 15, 19 6 3 3,073,385 FINNED TUBES Martin Charles Peters, New York, N.Y., assignor to Babcock & Wilcox Limited, London, England, a company of Great Britain Filed Nov. 18, 1958, Ser. No. 774,658 Claims priority, application Great Britain Nov. 19, 1957 8 Claims. (Cl. 165181) This invention relates to finned tubes of the kind provided with a fin or fins affording longitudinally spaced, transversely extending heat exchange surfaces and to heat exchangers formed of such tubes.

In heat utilising plant in which a fluid heat carrier is? forcibly circulated in series through a heat source and a heat exchanger wherein the fluid is cooled through a limited temperature range, conflicting requirements arise. On the one hand the power required to effect circulation of the fluid flowing over the extended surfaces thereof must be maintained at a value as low as possible, and on the other hand an effective heat exchange must be achieved with a heat exchanger of compact form.

An object of the invention is the provision of finned tubes of improved form making possible the construction of heat exchangers having favourable characteristics as regards heat transfer and pressure drop of the fluid flowing over the outer surfaces thereof.

The present invention includes a finned tube with longitudinally spaced, transversely extending fin surfaces provided by a fin or fins, wherein the or each fin includes a main part extending from the tube and formed with parallel or substantially parallel sides and a border part tapering from the main part towards an edge in a manner adapted to reduce eddying during its passage over the faces of the fin of fluid which flows-transversely of the tube and directly towards gaps between opposed fin surfaces.

The invention also includes a finned tube with longitudinally spaced, transversely extending fin surfaces provided by a helical fin, wherein the fin includes a main part extending from the tube and formed with parallel or substantially parallel sides and a border part tapering from the main part towards the edge of the fin in a manner adapted to reduce eddying during its passage over the faces of the fin of fluid which flows transversely of the tube and directly towards gaps between opposed fin surfaces.

The invention moreover includes a finned tube with longitudinally spaced, transversely extending fin surfaces provided by a multiplicity of fins each of which includes a central part extending from the tube and formed with parallel or substantially parallel sides and border parts at opposite sides of the central part each tapering from the central part towards the adjacent fin edge in a manner adapted to reduce eddying during its passage over the faces of the fin of fluid which flows transversely of the tube and approaches the said edge in a direction parallel or approximately parallel to the sides of the central part.

The invention also includes a finned tube with longitudinally. spaced, transversely extending fin surfaces provided by a multiplicity of fins each of which includes a central part extending from the tube and formed with parallel or substantially parallel sides and a border part extending around the central part at the opposite sides and at the outer end thereof and tapering from the central part towards the adjacent fin edge in a manner adapted to reduce eddying during its passage over the faces of the fin of fluid which flows transversely of the tube and approaches the fin in a direction parallel or approximately parallel to the sides of the central part.

The invention furthermore includes a finned tube with longitudinally spaced, transversely extending fin surfaces provided by a multiplicity of fins each of which is lobe shaped and includes a central part extending from the tube and formed with parallel or substantially parallel sides and a border part, extending from the tube around the central part, of substantial width and tapering from the central part to a narrow or knife edge.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a cross-section of a heat exchanger tube provided with an external helical fin,

FIGURE 2 is in its upper half, a side elevation of a length of the finned heat exchanger tube of FIGURE 1 in section on a plane through its axis, and, in its lower half, an external side elevation of the length of the finned tube,

FIGURE 3 is an elevation of part of a heat exchanger tube bank in section on a plane at right angles to the tube axes,

FIGURES 4 to 7 are cross-sectional views, to a larger scale than FIGURES 1 and 2, of helical fins of different forms,

FIGURE 8 is a cross-section of a heat exchanger tube provided with a multiplicity of rectangular fins,

FIGURE 9 is a side elevation of a length of the finned tube of FIGURE 8,

FIGURE 10 is a side view to a larger scale of a rectangular fin intended for welding on to a tube surface,

FIGURE 11 is an end view of the fin shown in FIG- URE 10,

FIGURE 12 is a side view of a modified form of rectangular fin,

FIGURE 13 is a cross-sectional view of another modified form of rectangular fin,

FIGURE 14 is a side elevation of a length of a heat exchanger tube provided with rectangular fins arranged obliquely to the tube axis,

FIGURE 15 is a cross-section of a heat exchanger tube provided with a multiplicity of lobe-shaped fins,

FIGURE 16 is a side elevation of a length of the finned tube of FIGURE 15,

FIGURE 17 is a side view to a larger scale of a lobeshaped fin intended for welding on to a tube surface,

FIGURE 18 is a view of the lobe-shaped fin in section on the line XVIII-XVIII of FIGURE 17,

FIGURE 19 is a view of the lobe-shaped fin in section on the line XIX-XIX of FIGURE 17, and

FIGURE 20 is a View of a modified form of lobeshaped fin.

Referring to FIGURES 1, 2 and 3 of the drawings, a heat exchanger tube 1 which is adapted for the transfer of heat from a gaseous fluid flowing past the exterior of the tube generally perpendicularly to the tube axis to a fluid caused to flow along the interior of the tube is provided with a single helical fin 2 adapted to increase the rate of heat withdrawal from the gaseous fluid. The inner edge 3 of the fin is welded to the tube surface.

The fin extends radially outwardly from the tube surface to a uniform height and is uniformally closely pitched along the tube. As shown by FIGURES 2 and 4, the outer border part 4 of the fin is symmetrically bevelled. The bevelled border part is at the most one third of the height of the fin. In the bevelled part of the fin a narrow outer edge face 5 is joined to the side faces 6 of the fin by straight bevel surfaces 7 which chamfer the respective outer corners between the side and edge faces of the fin. The thickness of the fin between the two side faces 6 thereof is small in relation to the height of the fin. I

A number of such finned tubes may be arranged parallel to one another in a bank with the tubes of adjacent rows in staggered relationship, the tubes extending in a direction normal to the general direction of gaseous fluid flow through the bank. FIGURE 3 indicates such an arrangement of rows 8, 9, 10 and 11 of parallel tubes 1 in a tube bank, where each tube extends in a direction normal to the general direction, indicated by arrows 12, of gas flow through the bank, and each row of tubes extends in a plane normal to the general direction of gas flow.

A mild steel fin suitable for a mild steel heat exchanger tube of 2 inch outside diameter, 1% inch internal diameter, is in one specific and useful construction /8 inch in height and about 0.08 inch in thickness between the side faces 6; the bevelled part 4 is inch in height and in the bevelled part the bevel surfaces 7 joining the narrow edge face 5 to the fin side faces 6 are at angles of 10 with the axis of the cross-section of the fin. Finned tubes of these dimensions are intended for arrangement in a tube bank as indicated with reference to FIGURE 3, with a distance between the centres of adjacent tubes in the same row of 4 /2 inch and a perpendicular distance between the planes of adjacent rows of 3 inch.

The fin, before application, is a strip formed by rolling with one edge symmetrically bevelled. The strip is heated to facilitate its being bent in its own plane, fed towards the tube, and progressively wound onto the tube while being welded thereto, weld metal being deposited, under the protection of an argon atmosphere, from a weld rod applied to the fin-tube junction at that side thereof which is accessible during the winding-on of the strip. It may be expected that the applied fin will, in consequence of its having been bent in its own plane, have, in cross-section, a slight radially outward taper.

In the operation of the heat exchanger, hot gas which in its flow sufiiciently approaches to the surface of the tube 1 enters the passages between adjacent convolutions of the helical fin 2. The gas flow is met by the tapered border parts of the fin and is thereby led into the passages between the fin convolutions without violent changes in direction. As a result, the formation of eddies in the gas as it enters the passages between adjacent fin convolutions is reduced or minimised. Moreover, gas leaving a passage between adjacent fin convolutions passes through a final part of the space which, between opposed bevelled fin parts in the adjacent fin convolutions, is diverging and is favourable to the conservation of flow energy in the gas. The bevelled part of a fin constitutes at the downstream side of a tube a trailing edge to the fin favourable to the discharge with only little eddy formation from the neighbourhood of a tube surface of gas which flows over the fin surfaces As a result the loss of pressure in the fluid flowing through the bank of tubes is much reduced.

The area of 'the fin surface available for receiving heat from the gases suffers, of course, a reduction due to the chamfering of the fin outer edges; however, this reduction is only very small. Since the fin is of substantially constant thickness over most of its height, the difference between the temperature of the metal at the fin tip and that of the tube wall is small, with the result that high temperature difference between the fin metal and the gas conducing to rapid heat exchange is maintained over most of the height of the fin. Moreover, thanks to the convergent entries between adjacent convolutions of the.

helical fin the gas is able readily to penetrate between the convolutions so that the surfaces of the convolutions are actively scrubbed. As a result the finned tube possesses excellent heat transfer properties.

In the modification indicated in FIGURE 5, the bevelled part 4 of the fin 2 has a bevel surface 21 on one side only of the fin.

If desired, the fin bevelled part, whether symmetrically or unsymmetrically tapered, may taper to a knife edge.

In the modification indicated in FIGURE 6, the outer border part 4 of the fin 2 is symmetrically bevelled by curved surfaces 22 which merge into the respective side faces 6 of the fin and merge into one another at the radially outer edge of the fin. In the modification indicated in FIGURE 7, the outer border part 4 of the fin 2 is bevelled on one side only by a curved surface 23 which merges into one side face 6 of the fin and approaches perpendicularly the other side face 6 of the fin at the radially outer edge of the fin.

If the tube and fin are of copper, the fin may be secured to the tube by brazing.

Referring to FIGURES 8 to 11, longitudinally spaced transversly extending fin surfaces for a heat exchanger tube 1 adapted for the transfer of heat from a gaseous fiuid flowing past the tube generally perpendicularly to the tube axis to fluid flowing within the tube are provided by a multiplicity of fins 31 rectangular in elevation arranged on the tube in circumferential rows or sets. Each fin set consists of six fins arranged in a plane perpendicular to the tube axis and disposed equiangularly around the tube, the fin sets are uniformly spaced along the tube, and the fins of alternate sets are staggered with respect to the fins of the remaining sets. All the fins have the same dimensions and are similarly attached to the tube surface.

Each fin is attached to the tube surface with an axis 32 thereof radial to the tube. The width of the fin measured perpendicular to this axis is greater than its height measured along said axis. Except in parts 33 bordering the two edges parallel to the axis, the fin is of uniform thickness small in relation to its width or height. The two border parts 33 are similar and each border part tapers symmetrically and uniformly from the central part 34 of uniform thickness to a narrow edge face. The width of each border part 33 is appreciably less than one third of the width of the fin.

Mild steel fins may be formed by cutting from a strip having the cross-sectional shape of the fins and produced by rolling. Such fins may be cheaply manufactured. They may be applied by electric resistance welding to a mild steel tube by the automatic machine described in the in the copending application of P. Adamson et al., Serial No. 798,471 filed March 10, 1959, now Patent No. 3,014,120 granted December 19, 1961, by the common assignee by which machine two diametrically opposite fins are simultaneously welded into position while held by respective reciprocable welding heads, the tube is turned through 60, a second pair of fins is attached to the tube by the same welding heads, the tube is turned through a further 60, a third pair of fins is attached to the tube to complete a set of six co-planar fins, the tube is turned through 30 and moved axially the distance of the desired pitch between fin sets, and the procedure is repeated. The staggering of the fins in adjacent sets permits the machine to be designed for operation with a relatively small pitch between fin sets.

Mild steel fins of the form described for application to a mild steel tube of 2 inch outside diameter, 1% inch internal diameter, are preferably of the following dimensions: Height inch, width 1 inch, thickness of central part inch, width of tapered border part 7 inch, angle of bevel 10 to the central plane of the fin, and in the array of fins electrically resistance welded to the tube the pitch between adjacent fin sets is preferably Ms inch. It will be observed that adjacent fins of a fin set are narrowly spaced apart at their radially inner ends. The finned tubes are preferably arranged in a tube bank in staggered rows with a distance of 4 /2 inches between adjacent tubes in the same row and a distance of 3 inches between the planes of adjacent rows. Each tube is furnished by the fins with a large extended surface area per unit length of the tube. In each fin, the central part 34 of uniform thickness allows a large cross-sectional area for heat flow, whereby the temperature difference between the metal of the fin tip and the metal of the tube wall is maintained small so that there is a substantial difference between the temperature of the gas and the temperature of the fin throughout its height. In flowing past a tube the gas is able, thanks to the narrow edges of the fins and the convergent inlets to the passages therebetween, readily to penetrate between the fins and effectively to scrub their surfaces. As a result the fins impart to the tube excellent heat transfer characteristics.

Gas approaching the tapered edges of a longitudinal row of fins is confronted by the narrow edge faces of the fins and upon entering between adjacent fins flows through the converging part of the passage therebetween defined by the border parts of the fins. As a consequence eddying of the gas is lessened or minimised. Moreover, in passing out of a passage between adjacent fins of a longitudinal row, the gas flows through the divergent part of the passage defined by the border parts of the fins at the downstream edges thereof, the flow energy in the gas thereby tending to be preserved. At the downstream border part of a fin, the tapering shape thereof is favourable to gas discharge therefrom without much eddy formation. As a result the pressure drop in the gas flowing over a tube bank is greatly reduced.

In a modification, a third border part 35, destined to be at the radially outer part of the fin, may be symmetrically and uniformally tapered, as indicated in FIGURE 12, according to the same shape as that of the border parts 33.

Each tapered border part may, if desired, taper to knife edges instead of to edge faces of finite area.

In other modifications, each tapered border part tapers symmetrically by means of a pair of curved surfaces, so that .a cross section of the fin has the form indicated by FIGURE 13, for example; or unsymmetrically by means of a single straight or curved surface.

Ilf in a heat exchanger the gas is to flow past a heat exchanger tube in a direction generally obliquely to its axis, the fins may be applied to the tube surface with an appropriate orientation so that their central planes are parallel to the gas flow direction. Referring to FIGURE 14, for example, staggered sets of six co-planar fins, each fin as shown in FIGURES l0 and 11, are obliquely arranged on a heat exchange tube, the plane of each set of six fins making an angle of 45 to the tube axis.

FIGURES 15 to 19 refer to a heat exchanger tube 1 provided with a multiplicity of similar fins 41 each of which, like the fin of FIGURE 12, has a central part 42 of uniform thickness bordered on all sides except the tube surface side by tapering parts 43, but in FIGURES 15 to 19 the fin is lobe-shaped.

Each lobe-shaped fin 41 in elevation is a major segment of a circle, defined by a straight edge 44, an arcuate edge 45, and short curved edge parts 46 merging into the straight edge part and the .arcuate edge part. The central part 42. of the fin is of uniform thickness, is circular and concentric with the arcuate edge 45 and the straight edge 44 runs close to the central part 42 of uniform thickness. Radially outwardly (:as regards the fin) from the central part 42 the fin border part 43 tapers symmetrically and uniformly to a narrow edge face 47. Such fins may be formed from blanks of sheet metal by cold forging.

The dimension of such :a lobe-shaped fin, intended for attachment to :a heat exchange tube of 2 inch outside diameter, 1% inch internal diameter, may be as follows: Radius of arcuate edge 1% inch, radius of central part 4 inch, thickness of central part 0.08 inch, distance of straight edge from central part A inch, taper of border part to central plane of fin.

During the welding operation, the fin is applied to the tube so that the centre of the straight edge 44 engages the tube surface; for the usual case, the fin will be perpendicular to the tube axis. Normally, electric resistance welding will be employed, the fin being mounted for the purpose of welding in :a hand-held tool or being fed by hand into the reciprocable head of a semi-automatic machine.

The fins are arranged in circumferential rows or sets of oo-planar fins, adjacent fins of a set being narrowly spaced apart at their radially inner ends, and in an array which, as shown in FIGURES and 16, is the same as the array of fins shown in FIGURES 8 and 9.

If desired, the fins may be arranged in circumferential rows or sets of co-planar fins of which the planes are parallel to one another and oblique to the tube axis.

In a modification, the lobe-shaped fin is formed, instead of with a straight edge 44, with a slightly curved edge 48 adapted to the curvature of the tube surface; such modification is illustrated in FIGURE 20.

I claim:

1. A finned tube having :a multiplicity of longitudinally spaced transversely extending fins positioned on its exterior surface and arranged for the flow of a heat transfer fluid passing over said tube and between said fins, each of said fins comprising a main part and a border part and being completely spaced apart from the adjacent fins throughout its surface, the main part of each of said fins formed by a pair of parallel sides extending from and attached to the tube along an edge thereof, all of the sides of the main parts of said fins being in parallel relationship with each other, the border part of said fins providing a substantial proportion of the surface thereof, said border part extending along at least a portion of the edge of said main part in the same plane therewith and tapering therefrom to an edge of said fin in a manner adapted to reduce eddying of the fluid during its flow between the sides of said fins which flow is oriented transversely of the tube to pass between the fins in a direction generally parallel to the sides of the main part of said fins.

2. A finned tube having a multiplicity of longitudinally spaced transversely extending fins positioned on its exterior surface and arranged for the flow of a heat transfer fluid passing over said tube and between said fins, each of said fins comprising a rectangular shaped central part and a pair of border parts and being completely spaced apart from the adjacent fins throughout its surface, the central part of each of said fins formed by a pair of parallel sides extending from and attached to the tube along an edge thereof, all of the sides of the central parts of said fins being in parallel relationship with each other, the border parts of said fins providing a substantial proportion of the surface thereof, said border parts arranged along opposite edges of said central part and extending in the same plane therewith and tapering therefrom towards the adjacent edge of said fin in a manner adapted to reduce eddying of the fluid during its flow between the sides of said fins which flow is oriented transversely of the tube to pass between the fins in a direction generally parallel to the sides of the central part of said fins.

3. A finned tube having a multiplicity of longitudinally spaced transversely extending fins positioned on its exterior surf-ace and arranged for the flow of a heat transfer fluid passing over said tube and between said fins, each of said fins comprising a rectangular shaped central part and a border part and being completely spaced apart from the adjacent fins throughout'its surface, the central part of each of said fins formed by a pair of parallel sides and four edges with the sides extending from and attached to the tube along one of the edges thereof, all of the sides of the central parts of said fins being in parallel relationship with each other, the border part of said fins providing a substantial proportion of the surface thereof, said border part located along the other three edges of said central part and extending in the same plane therewith and tapering therefrom to the adjacent edge of said fin in a manner adapted to reduce eddying of the fluid during its flow between the sides of said fins which flow is orineted transversely of the tube to pass between the fins in a direction generally parallel to the sides of the central part of said fins.

4. A finned tube having a multiplicity of longitudinally spaced transversely extending fins positioned on its exterior surface and arranged for the flow of a heat transfer fluid passing over said tube and between said fins, each of said fins being lobe shaped and comprising a central part and a border part and being completely spaced apart from the adjacent fins throughout its surface, the central part of each of said fins being circular in shape and having a pair of parallel sides extending from and attached to the tube along an edge thereof, all of the sides of the central parts of said fins being in parallel relationship with each other, the border part of said fins providing a substantial proportion of the surface thereof, said border part located along the periphery of said central part which is out of contact with said tube and extending in the same plane therewith and tapering therefrom to a narrow knife-like edge, said fin shaped in a manner adapted to reduce eddying of the fluid during its flow between the sides of said fins which flow is oriented transversely of the tube to pass between the fins in a direction generally parallel to the sides of the central part of said fins.

5. A finned tube as claimed in claim 2 wherein the fins are arranged on the tube in circumferential rows of co-planar fins.

6. A finned tube as claimed in claim 5 wherein each fin row consists of pairs of diametrically opposite fins.

7. A finned tube as claimed in claim 5 wherein the fins in adjacent fin rows are staggered.

8. A finned tube as claimed in claim 5 wherein the planes of the circumferential rows of fins are oblique to the axis of the tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,764,603 Bell June 17, 1930 1,916,574 Key July 4, 1933 1,997,502 Von Seld et a1. Apr. 9, 1935 2,201,024 Brown May 14, 1940 2,554,661 Clancy May 29, 1951 

1. A FINNED TUBE HAVING A MULTIPLICITY OF LONGITUDINALLY SPACED TRANSVERSELY EXTENDING FINS POSITIONED ON ITS EXTERIOR SURFACE AND ARRANGED FOR THE FLOW OF A HEAT TRANSFER FLUID PASSING OVER SAID TUBE AND BETWEEN SAID FINS, EACH OF SAID FINS COMPRISING A MAIN PART AND A BORDER PART AND BEING COMPLETELY SPACED APART FROM THE ADJACENT FINS THROUGHOUT ITS SURFACE, THE MAIN PART OF EACH OF SAID FINS FORMED BY A PAIR OF PARALLEL SIDES EXTENDING FROM AND ATTACHED TO THE TUBE ALONG AN EDGE THEREOF, ALL OF THE SIDES OF THE MAIN PARTS OF SAID FINS BEING IN PARALLEL RELATIONSHIP WITH EACH OTHER, THE BORDER PART OF SAID FINS PROVIDING A SUBSTANTIAL PROPORTION OF THE SURFACE THEREOF, SAID BORDER PART EXTENDING ALONG AT LEAST A PORTION OF THE EDGE OF SAID MAIN PART IN THE SAME PLANE THEREWITH AND TAPERING THEREFROM TO AN EDGE OF SAID FIN IN A MANNER ADAPTED TO REDUCE EDDYING OF THE FLUID DURING ITS FLOW BETWEEN THE SIDES OF SAID FINS WHICH FLOW IS ORIENTED TRANSVERSELY OF THE TUBE TO PASS BETWEEN THE FINS IN A DIRECTION GENERALLY PARALLEL TO THE SIDES OF THE MAIN PART OF SAID FINS. 